It is generally observed that a silicon wafer cut out from a dislocation-free silicon monocrystal ingot grown by the Czochralski (CZ) method has a crystal defect called COP (Crystal Originated Particle). The COP is caused by voids introduced during the ingot growing process. When voids are exposed on the surface of the silicon wafer to form pits or, even if not exposed on the surface, when they are contained in the surface layer of the silicon wafer (i.e., when the COP is contained in a device activation area), they become a factor of causing a defect in the device properties, such as a defect in withstand voltage in the gate oxide film or a defect in leak characteristic.
In view of the circumstances described above, in order to remove the COP which is exposed on the surface of the silicon wafer or included in the surface layer, a heat treatment (annealing) is conducted in a nonoxidizing atmosphere such as a hydrogen atmosphere or an argon atmosphere at a high temperature of approximately 1200° C. (e.g., Japanese Patent Application Laid-Open Publications No. 58-85534 and No. 4-167433).
The above heat treatment (annealing) actually reduces or eliminates the COP exposed on the surface of the silicon wafer or contained in the surface layer by diff-using oxygen from the wafer surface layer to the outside.
But, when the silicon wafer having boron as a dopant is annealed in a hydrogen atmosphere, not only oxygen but also boron is diffused from the wafer surface layer to the outside. Thus, there is a drawback that a resistivity in the vicinity of the wafer surface layer becomes different from the initially assumed one.
In this connection, Japanese Patent Application Laid-Open Publication No. 10-144698 discloses a method of treating a silicon wafer containing boron in an argon atmosphere with almost no change in a distribution of boron based on a discovery made by the inventors that when the silicon wafer containing boron is annealed in the argon atmosphere, boron is hardly diffused to the outside but only oxygen is selectively diffused to the outside.
However, the invention described in Japanese Patent Application Laid-Open Publication No. 10-144698 is strictly conducted as a pretreatment of epitaxial growth. In practice, it cannot fully uniformize the boron concentration in the thickness direction of the wafer surface layer.